The present invention relates to a circuit and a method for detecting an overvoltage condition and for protecting a transistor in response thereto. Detection of the overvoltage condition may be integrated with protection for an overcurrent condition as well as excessive heat, and is accomplished by sensing a reverse base current. The safe operating area ("SOA") of the transistor may be extended in response to the detected condition by reducing the externally generated base-emitter current.
Transistor operation in a forward active state is usually limited to an SOA that is defined in the collector voltage/collector current plane, an example of which may be seen in FIG. 1. Operation outside the SOA can result in transistor failure from high temperatures caused by excessive power dissipation.
With reference to FIG. 1, the line A-B represents the upper limit of the transistor's current carrying capability and the line B-C illustrates the basic thermal limitation associated with the maximum allowable junction temperature, i.e., the first breakdown condition. The line C-D-E illustrates the limit imposed by the second breakdown discussed below, and the line E-F is the collector-emitter breakdown voltage BV.sub.CEO, i.e., the maximum voltage that can be applied from the collector to the emitter with the transistor's base open circuited (I.sub.B =0). The present invention is directed to extending the SOA limits defined by the line C-D-E-F as suggested by dashed line C-G-H in FIG. 1; that is, in the areas limited by second breakdown and the BV.sub.CEO.
The second breakdown limitation may be the result of several mechanisms. One mechanism is localized thermal run-away; that is, non-homogeneous current densities in the transistor that cause localized "hot spots". This mechanism can be controlled by keeping the current density uniform across the emitter. To this end, the transistor can be modified to include multiple emitters with small ballasting resistors.
The second mechanism, the one to which the present invention is addressed, is related to a phenomena known as avalanche multiplication that may be more clearly understood with reference to FIG. 2. With reference to FIG. 2, assume transistor Q1 is operating with a collector current I.sub.c that is much greater than an externally supplied base (biasing) current I.sub.B and that the collector to base voltage V.sub.CB is increasing. As the collector electrons pass through the collector-base depletion region, they gain energy from the electric field (VCB). At some voltage, a few of the collector electrons have enough energy to generate electron-hole pairs when they collide with a lattice atom. The number of collector electrons that generate electron-hole pairs increases exponentially with the electric field as is shown in Equation (1) below. EQU M.apprxeq.exp ((V.sub.CB -K.sub.1)/K.sub.2) (1)
where K.sub.1 and K.sub.2 are constants dependent on the construction of the transistor, and PA1 where M.ltoreq.1.
The generation of the electron-hole pairs causes the collector current to increase beyond that associated with the base current to the point of transistor failure.
Avalanche multiplication increases collector current in two ways, i.e., directly due to the new electrons, and indirectly due to the increase in base current from the new holes, assuming a constant external base current. In other words, the new electrons are added to the collector current I.sub.c while the new holes go to the base and are added to the externally base current I.sub.B. At BV.sub.CEO, the component of base current generated by the new holes is sufficient to keep the transistor on without any base current from an external source. Above BV.sub.CEO, the base current I.sub.B actually reverses direction from that shown in FIG. 2. As may be seen in FIG. 1, the situation where voltage is increasing and current is increasing (due to avalanche multiplication) will quickly drive transistor operation out of the SOA.
Nevertheless, situations exist where it is desirable to have a transistor that is able to operate outside of its traditional SOA to its BV.sub.CES (or BV.sub.CBO). For example, there are applications where a transistor is normally operating at voltages only slightly below its BV.sub.CEO and there may be disturbances on the high voltage supply which would take the collector-emitter voltage of the transistor above BV.sub.CEO. The transistor may not have to supply full load current during such a disturbance, but it is desirable that the transistor survive. A transistor with this ability may be useful, for example, in a prior art high voltage linear voltage regulator such as illustrated in FIG. 2.
A transistor can be operated at voltages above BV.sub.CEO if its collector current is sensed and controlled. This may be accomplished by using a high voltage, voltage sensing element. However, unless the collector current is limited to a small value, the need to dissipate power from such an element will present space and/or heat problems. For example, a high voltage resistor divider network (or a combination of a resistor network and a Zener diode) could be used, but the area of the resistor would be quite large and would not likely find application in most integrated circuits where space is typically a major concern.
Accordingly, it is an object of the present invention to provide a novel overvoltage detection circuit and method.
Another object of the present invention is to provide an novel circuit and method for extending the SOA of a transistor so that the transistor can safely operate with voltages above its BV.sub.CEO.
It is a further object of the present invention to provide a novel circuit and method for controlling the collector current in a transistor so that its SOA is extended.
It is yet a further object of the present invention to provide a novel integrated circuit for detecting the magnitude and direction of a transistor's base current.
It is another object of the present invention to provide a novel circuit and method for extending a transistor's SOA by reducing externally supplied base current responsive to detection of reverse base current above a predetermined threshold.
It is yet another object of the present invention to provide a novel circuit and method for detecting excessive voltage, current and heat in an integrated circuit transistor.
Yet a further object of the present invention to provide a novel circuit and method for protecting an integrated circuit transistor against excessive voltage, current and heat.
Yet still a further object of the present invention to provide a novel high voltage linear regulator and method.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.